Laser devices having an active medium of small length like disk lasers, microchip lasers or VCSELs (Vertical Cavity Surface Emitting Laser) or surface emitting organic material lasers emit an optical power which is substantially proportional to the emission area. For several applications, such laser devices with a relatively high optical power are required. However, a problem occurs by increasing the emission area because the spatial coherence of the emitted beam decreases. This loss of spatial coherence results from the excitation of several transverse modes. This is schematically represented in FIGS. 1A and 1B. On FIG. 1A is represented a disk laser 2 emitting substantially only the fundamental mode, the wave fronts 4 of which are shown. On FIG. 1B is represented a disk laser 6 having a wider emitting surface and emitting several transverse modes, the resulting distorted wave front 8 being schematically shown.
A solution is proposed by the company Novalux whereby the control of the transverse mode spectrum of a wide area VCSEL is achieved by an external mirror. Such solution is not compatible with low cost batch manufacturing technologies. Another solution is proposed by the company Alight whereby the outer multilayer mirror of a VCSEL is pierced by deep holes distributed in the form of a ring surrounding the circular emission area and providing a photonic crystal transverse feedback controlling the emitted transverse mode spectrum. Such solution is difficult to realize technologically and represents a strong perturbation of the whole VCSEL structure.
Compact microchip Q-switched lasers delivering at least millijoule pulses in a single transverse mode are desired for many applications [Ref. M. Weinrotter, H. Kopecek and E. Wintner, Laser Ignition of engines, Laser Physics, 15(7), 947, 2005]. However, the short length of the microchip resonator limits the size of the fundamental mode to several tens of μm, while the energy per unit area is limited to a fraction of the saturation parameter of the material, typically in the order of or less than a few J/cm2 [Dong J., Opt. Rev. 12(3), 170-178, 2005]. Consequently, mode size in the order of several hundreds of μm is required, but this is impossible to obtain with current state-of-the-art microchip lasers.
Selective laser mirrors comprising a multilayer and a resonant grating next to the latter or integrated into the latter are disclosed in document WO 01/09650 A2 and document “Polarizing grating mirror for CW Nd:YAG microchip laser” by F. Pigeon, O. Parriaux, Y. Ouerdane, A. V. Tishchenko, IEEE Photonics Technology Letters, Vol. 12, June 2000, pp. 648-650. These documents associate one multilayer and one waveguide grating submirrors in a destructive interference scheme to induce a reflection coefficient difference between the two polarizations emitted by a laser so as control the emitted polarization by filtering out the polarization which is coupled to a waveguide mode of the grating waveguide. These documents describe a polarization selection mechanism which relies upon the excitation of one waveguide mode in one direction by means of a 1-D grating pattern. There is no teaching in these documents as to how to achieve polarization independent transverse mode control of a wave emitted by a laser.